There are a variety of currently available web feeding machines. These machines match speed between the web feeder and a web take-up device such as a printer. However, existing speed matching techniques do not perform well in applications requiring high throughput speeds or widely varying speeds.
Existing machines often rely upon a weighted bar or dancer that rides on the web between the web feeding device and the web take-up device. The dancer pivots to apply consistent weight to the loop of web material formed between the web handling devices, which holds the web in place. A sensor, such as a variable resistor, is further employed to detect the position of the dancer and, as a result, the amount of free web material in the loop between the web feeding device and the web take-up device. In this manner, the web feeder's rate can be adjusted to supply web material at a rate consistent with the demands of the take-up device. This weighted bar approach may be adequate for some applications. However, the loop of web material becomes unstable at high speeds. Also, when used with take-up devices such as impact printers or bursters, which have widely varying speeds, this prior art arrangement is quite inadequate since the bar will bounce during abrupt speed changes. Accordingly, an object of the present invention is to match speed between a web feeding device and a web take-up device in applications with widely varying speed requirements.
The weighted bar device additionally places substantial strain on the pin feed holes in the web material. This is particularly a problem when handling light weight paper. Even with the weight of the dancer counterbalanced, the speed changes in a typical web feeding operation cause inertial forces that distort the pin feed holes. This tension complicates the paper steering operation, requiring exact alignment and trim. Accordingly, it is an object of the present invention to provide a technique for joining a web feeding device and a web take-up device that does not require the use of dancers or the careful alignment of web material and machinery.
Another technique for speed matching involves the use of a "free loop," where the web feeder is connected directly to the web take-up device while the intervening web material hangs freely between the two apparatuses. In this case a sensor, such as an optical or ultrasonic detector positioned under the loop, is typically employed to sense the depth of the web material loop. The web feeding machine, in turn, uses the measured loop depth to control the web feeding rate. However, this approach is prone to instability whenever operating conditions cause the loop shape to deform. Further, if the feed rate drops sharply, the unrestricted loop of web material may crease, twist, or completely derail from the web feeder or the take-up device, requiring operator intervention and interruptions to the web handling process. This approach also suffers from difficulties where the loop shape deforms in response to environmental conditions such as external air currents.
Accordingly, it is an object of the present invention to provide a consistent free loop shape between two web handling devices to permit accurate sensing and speed matching.